Recent years have seen a significant reduction in the size and weight of mobile information terminals such as cellular phones and laptop computers, and there is a need for a battery with a higher capacity as their power supply. Although nonaqueous electrolyte secondary batteries, such as lithium (Li) ion batteries, which have particularly high energy density among secondary batteries, have increasingly high capacities, the above need has yet to be satisfied. Recently, nonaqueous electrolyte secondary batteries have been used in a broader range of applications due to their characteristics. For example, nonaqueous electrolyte secondary batteries have increasingly been used not only in mobile applications such as cellular phones, portable computers, PDAs, and portable music players, but also in medium to large battery applications, including power applications such as electric tools, electric bicycles, electric vehicles (EVs), and hybrid electric vehicles (HEVs and PHEVs) and storage applications such as backup power supplies and power storage systems.
With the increasing applications of nonaqueous electrolyte secondary batteries, nonaqueous electrolyte secondary batteries having an increasingly wide variety of structures have been developed. Examples of such nonaqueous electrolyte secondary batteries include cylindrical batteries including a rolled electrode assembly composed of electrodes and a separator rolled together, rectangular batteries including a rolled electrode assembly pressed and inserted into a rectangular casing or covered with a laminate casing, rectangular batteries including an electrode assembly composed of electrodes and separators stacked on top of each other and inserted into a rectangular casing or covered with a laminate casing, and button batteries.
The development of nonaqueous electrolyte secondary batteries of various shapes has complicated the manufacturing processes of nonaqueous electrolyte secondary batteries. This leads to decreased productivity, including increased manufacturing costs of nonaqueous electrolyte secondary batteries and an extended lead time. Accordingly, it is essential to develop a technique for simplifying the manufacturing processes of an increasingly wide variety of nonaqueous electrolyte secondary batteries.
Polyolefin fine porous films, which are used as separators in nonaqueous electrolyte secondary batteries, increase the cost of battery materials and involve a complicated manufacturing process and an extended lead time due to the need for the steps of stacking and rolling together with electrodes. Accordingly, the inventors have made various attempts to develop a battery including no conventional polyolefin fine porous film, i.e., a separatorless battery.
Conventional nonaqueous electrolytes secondary batteries include a separator made of an insulating porous material through which lithium ions pass between positive and negative electrode plates. The separator is a fine porous film of a polyolefin such as polyethylene or polypropylene. To develop a nonaqueous electrolyte secondary battery including no such separator, i.e., a separatorless battery, a porous layer of a low-cost material must be directly formed on a surface of an electrode to replace a polyolefin fine porous film.
PTL 1 below discloses an invention related to a nonaqueous electrolyte secondary battery that includes a negative electrode including a current collector and a negative active material coating layer disposed thereon, a positive electrode including a current collector and a positive active material coating layer disposed thereon, a separator, and a nonaqueous electrolyte. A porous protective film having a thickness of 0.1 to 200 μm and containing a resin binder and fine solid particles of alumina or silica powder having particle sizes in the range from 0.1 to 50 μm is formed on a surface of the positive or negative active material coating layer. PTL 2 below discloses an invention related to a separator-electrode-integrated storage element for electrochemical devices that includes an electrode and a separator integrated with a surface thereof. The separator is a porous layer containing fibers formed by electrospinning.